There are several modules that will very quickly drain your cap when they activated for a longer period of time. These include your microwarpdrive, and shield/armor reps.
There rarely is a need to keep these modules activated constantly though. You use the WMD for one or two cycles to quickly cover some distance but then you turn it off as it increases the damage you take due to increased signature size. You use the reps for a single cycle every so often to repair some damage. Running them when you have no, or very little, damage is a waste of cap.